The present invention relates to a polyvinyl alcohol product and a multi-layer product containing the same.
Polyvinyl alcohol (hereinunder referred to as "PVA"), which is a resin excellent in a chemical resistance and physical properties, especially, in an oxygen gas barrier property in a dry phase, is used in a broad industrial field. PVA, however, has a crystal melting point (hereinunder referred to as "Tm") higher than the decomposition beginning temperature (hereinunder referred to as "Td"). It is, therefore, impossible to utilize a simple melt-extrusion process unlike polyethylene, polyvinyl chloride and polypropylene, and it is necessary to make pellets impregnated with a plasticizer such as glycerin and low-molecular polyethylene glycol or water, and to extrude the pellets by an extruder equipped with special melting, kneading and defoaming mechanisms (see Poval, p.377, published by KOBUNSHI KANKOKAI on Apr. 25, 1984 and Encyclopedia of Polymer Science and Engineering (1989) vol. 17, pp. 186 to 187, published by John Wiley and Sons, Inc.)
Therefore, when PVA is industrially used, it is generally formed into films, sheets and fibers by flow casting or wet spinning after PVA is dissolved in water.
In order to facilitate the hot working of PVA, use as a vinyl alcohol-vinyl acetate copolymer with a view to reducing the saponification degree of PVA or a copolymer of vinyl alcohol and ethylene, etc. has been proposed, but this way is defective in that the oxygen gas barrier property of the product obtained is lowered.
As a composition of PVA and a metal salt, the following methods and products are disclosed in the literatures.
(1) A method of producing a stock solution for a polyvinyl alcohol spinning or extruding product, comprising the steps of: preparing an aqueous solution of 20 to 80 wt % of at least one metal chloride selected from the group consisting of zinc chloride, calcium chloride, aluminum chloride and magnesium chloride or preparing a mixed aqueous solution by mixing not more than 40 wt % of hydrochloric acid, nitric acid or a water-soluble organic compound with the said aqueous solution; and dissolving polyvinyl alcohol or a copolymer mainly composed of vinyl alcohol in the said aqueous solution or mixed aqueous solution (Japanese Patent Publication No. 43-29660 (1968)).
(2) A gas barrier product produced by film-forming a water-soluble high-molecular material with lithium halide added thereto, or impregnating or coating a porous material with a water-soluble high-molecular material with lithium halide added thereto (Japanese Patent Application Laid-Open (KOKAI) No. 49-45882 (1974)).
(3) A method of producing a polyvinyl alcohol polarized film, comprising the steps of: film-forming an aqueous solution of polyvinyl alcohol with a water-soluble metal halide dissolved therein by flowcasting; and uniaxially stretching the film at 90.degree. to 150.degree. C. (Japanese Patent Application Laid-Open (KOKAI) No. 56-25419 (1981)).
(4) A heating element for generating heat by water, produced by mixing 3 to 100 parts by weight of a water-soluble resin with 100 parts by weight of magnesium chloride (Japanese Patent Application Laid-Open (KOKAI) No. 60-120784 (1985)).
(5) A moldable desiccating agent comprising a deliquescent salt and a high-water-absorbing synthetic resin as the main ingredients (Japanese Patent Application Laid-Open (KOKAI) No. 61-141916 (1986)).
(6) An adhesive PVA hydrogel composition comprising polyvinyl alcohol and a divalent metal salt as well as water as the main ingredients (Japanese Patent Application Laid-Open (KOKAI) No. 1-230659 (1989)).
However, conventional compositions of PVA and a metal salt, and products produced therefrom cannot be said to have a sufficiently high gas barrier property and, especially, in an atmosphere of a high humidity, the gas barrier property of such a film is so greatly lowered that the film cannot be put to practical use as a food packaging film.
As a result of studies undertaken by the present inventor so as to solve the above-described problems, it has been found that a PVA product obtained by heat-treating and/or stretching the molded product of PVA with a specific amount of metal compound added thereto and a multi-layer product having at least one inner layer of a melt-molded product of PVA with a specific amount of metal compound added thereto can maintain a high gas barrier property even at a high humidity. On the basis of these findings, the present invention has been achieved.